Button, who was jailed for life with her obsessed nephew Simon Tannahill for bludgeoning her husband George to death so they could cash in on his £450,000 insurance, wrote a series of letters to her sister Susan Miller while on remand and later bail.

And while some of them have been seen by police and reported in the Chronicle before, Susan chose to keep a few private and they remained forgotten in a cupboard for nearly a decade.

But, after learning a campaign group wants her conviction reviewed, the 50-year-old decided to show a glimpse of how cunning her sister could be.

Revealed for the first time today, the letters, written while Button was bailed to her sister house’s in Preston, Lancashire, while she awaited her trial, reveal how she:

l told Susan to “think before she speaks” when talking to authorities and not to go “shouting her mouth off”

l branded the police corrupt

l maintained the officers had a vendetta against her

l attacked the police for their treatment of her dead husband, and

l continued to protest her innocence.

Debt-ridden Button was found guilty of encouraging her nephew, Simon Tannahill, to bludgeon her husband George Button to death as he walked his dog in West Rainton, County Durham, in March 2003.

The 44-year-old had run up debts of more than £200,000 before she masterminded the murder of her husband and is now more than six years into her life sentence.

Police initially thought Mr Button had been the victim of a hit and run, but tests by forensic scientists later revealed he’d been the victim of a bloody murder.

Susan, who brought-up Button, her youngest sister, always knew the truth and refused to lie for murderous sibling.

Button sent the letters to Susan, formerly of Low Fell, Gateshead, but now living in Ferryhill, County Durham, while she was on remand and later bailed to another sister’s house in Preston.

The letters have only ever been seen by Susan.

In one, written shortly after she was arrested in March 2003, Button wrote: “My impression of the law has changed greatly, I now feel that there is so much corruption, dishonesty and betrayal.

“I think [the police] have forgotten exactly what their job description says. I will never trust the police again., as long as I live.”

She continues: “It actually feels like a personal vendetta against me now. I feel like they know they have made a mistake, so they are desperate to turn people against me and discredit me.”

In another letter, ahead of Susan giving a statement to police, Button writes: “All I would ask of you Susan is to think before you speak. I know you mean well and are just trying to help me.

“But, if you really want to help me, you’ll just have to keep yourself to yourself, which I know is very hard.”

Three-months-later, Button wrote another letting, telling Susan not to “go shouting your mouth off to the anyone, friends, medical staff and police about this.”

In another letter, in June 2003, Button also lashes out at police and their treatment of her murdered husband. In a letter from last June she said: “They showed him no dignity, compassion or respect...they will pay badly for this.”

Throughout all her letters, the killer refers regularly to the police as b****ds.

Shortly before George’s funeral, Button wrote to her sister again telling her not to go as it was only for her and the couple’s daughter Laura to attend.

But Susan said it was a ploy to keep her away in case she said anything, as 300 people eventually attended the service.

Susan, who is battling cancer and has been ostracised by the rest of her family since she decided to speak out about her sister, said: “Some of the letters I’ve given to the Chronicle before but there were a few nobody has seen, particularly the ones about the police and about me keeping quiet.

“I’ve decided to show the rest of the letters now since there has been a call to review her conviction. In them, I’m told to keep quiet – she was nervous that I would land her in it – and she goes on about the police.

“She does go on about how she’s innocent, but Christina is very, very manipulative and cunning. My family don’t speak to me anymore, they tell people I’m dead, but I want to make sure Christina serves her time for what she did to George.”